1. Field of Invention
This invention is directed to a process for applying a color watermark to a digitally reproducible color image to enable the verification of the color image""s authenticity.
2. Description of Related Art
Watermarks have long been used in the printing industry to identify the source or origin of a document. Generally, a watermark appears as a faint pattern in an image, which is visible only when the original document is viewed in a particular manner. Unless a copyist had access to the watermarked paper, it would be difficult for the copyist to reproduce the document without revealing the document""s inauthenticity. That is, without the paper on which the original image was originally printed, the copy would be readily detectable. However, as people move away from the use of watermarked papers for cost and other practical reasons, it is still necessary to identify the source or origin of a document image.
The introduction of plain paper copies has resulted in a proliferation of paper copies of paper originals. A similar result is happening to electronic images, given the easy availability of digital scanners and the quick and widespread access to images throughout the Internet. It is now very difficult for the creator of an image to generate an electronic original that the creator can be assured will not be illegally copied and spread to third parties. A digital watermark aims to prevent that spread, by incorporating an identifying mark within the image that allows the source of the image in an electronic copy to be identified. It is important that the identifying mark not disturb or distract from the original content of the image, while at the same time allowing the source to be easily identified.
Watermark identification may be accomplished by embedding a watermark in a digital or printed page that will identify the owner of rights to the image. In the past, these images have been produced and delivered in hard copy. In the future, these images will be distributed mainly in digital form. Therefore, image identification will have to work for both hard copy and digital image forms.
Watermarking can take two basic forms: visible or perceptible, and invisible or imperceptible. Visible watermarks are marks, such as copyright symbols or logos, that are imprinted into the digital or printed image to be distributed. The presence of the watermark is made clearly visible in the image in a way that makes it difficult to remove the watermark without damaging the image. The presence of the visible watermark does not harm the usefulness of the image. However, visible watermarks may interfere with the image aesthetics. The visible watermark is also a potential target for fraud, in that it is possible for a fraudulent copier of the image to identify the location of the watermark and attempt to reproduce the image without the watermark.
Invisible watermarks are marks, such as copyright symbols, logos, serial numbers, etc., that are embedded into digital or printed images in a way which is not easily discernible to the unaided eye. At a later time, the information embedded in these watermarks can be derived from the images to aid identification of the source of the image, including the owner and the individual to whom the image is sold. Such watermarks are useful for establishing ownership when ownership of an image is in dispute. Such watermarks are less likely to be useful as a deterrent to the theft of the image.
While either or both visible or invisible watermarks are desirable in an image, they represent different techniques for either preventing copying or detecting copying. It is anticipated that document producers may wish to use both kinds of protection.
Previously, a number of patents and publications have disclosed watermarking or other digital information encoding techniques for use with documents. The patents and publications summarized below are all incorporated herein by reference in their entireties.
A method of embedding information into halftones was suggested by Tuhro, xe2x80x9cCounterfeit Detection Methodxe2x80x9d, Xerox Disclosure Journal, Vol. 20, No. 6, November/December 1995. This method makes slight deviations in the position of the halftone dots from cell to cell. Alternatively, the position of a line screen could be varied by small amounts to encode information. The decoding would be accomplished by laying a halftone screen that has no such deviations on top of the encoded image. The locations of the halftone cell deviations show up as a beat pattern. The difficulty of this method is that the information could easily be lost within the printer distortions. The goal would be to make the deviations as small as possible to avoid their being seen. This results in the deviations being hard to detect as well.
A second method is described in U.S. Pat. No. 5,706,099 to Curry. This method processes halftone dots, called serpentine dots, that are symmetric in shape. These dots are designed to fit well together when rotated and placed side by side. This enables different rotations of the dots to be placed across the page, encoding arbitrary information into the halftone pattern. From a visual inspection, the changing shapes of the symmetric patterns are not displeasing to the eye. This method suffers from the same problems as the method by Tuhro. In order to make sure that the information cannot be seen by the eye, the halftone cells need to be made as small as possible. As cell size is reduced, it becomes harder to detect cell orientation.
Yet another process for embedding information into an image is through the use of glyphs. As disclosed in U.S. Pat. No. 5,315,098 to Tow, glyphs are small, three to five pixel length lines in which the orientation of the line represents one of a small number of values. Thus, a stream of data can be represented by glyphs, where each line varying in orientation in order to provide an encoded view of the information. Glyphs differ from watermarks in that the glyphs encode information via some numerical method, while watermarks represent the actual image. The glyphs may be machine readable by means of human invisible characteristics of the print materials, such as their infrared reflectivity, their high-resolution spectral detail, their metameric spectral characteristics, their magnetization, or the like. These machine detectable materials may be incorporated into the same printing process that is employed for printing the human-readable rendering, such as by utilizing xerographic toners which have machine-recognizable, human-invisible characteristics, together with their usual visible characteristics of color, whiteness, blackness, transparency and opacity.
U.S. Pat. No. 5,337,361 to Wang et al. describes an information area that can overlay a graphic image and include information encoded in an error correctable, machine-readable format. This method allows recovery of the information despite distortions due to the underlying graphic image. The record may also represent the image by words similar in form to words in the image area. Both the image information and the graphic words can then be altered when an action regarding the record takes place. Wang et al. differs from the present invention in that it essentially provides a bar code encoding information other than the image that is desired to be seen.
xe2x80x9cCloaking Device for Top-Secret Faxesxe2x80x9d, Electronic Imaging Review, Center for Electronic Imaging Systems, University of Rochester, N.Y., Fall, 1995, Vol. 2, No. 3, Page 4, details software which supports an encryption method so that all the pages look the same: a random pattern of black and white dots filled with tiny black worms. The page is decrypted by placing a decryption key over the page. Text appears as either white lettering against a black background or dark lettering against a white background. Notably, in the images shown, the region where the image is located is discernible via changes in the random pattern of black and white dots.
Several articles referencing a method of adding a digital watermark are noted with respect to Digimarc Corp. xe2x80x9cDigimarc Corp. Announces New Copyright Protection Technology; Irremovable Signatures Protect Creative Property in the Digital Age,xe2x80x9d Jun. 28, 1995, Business Wire, describes a copyright protection system in which hidden information is available with an image. See also, xe2x80x9cDice and Digimarc File for Patents for xe2x80x98Scatter-Gunxe2x80x99 Electronic Watermark Technology,xe2x80x9d Oct. 4, 1995, Computergram International; xe2x80x9cHolographic signatures for digital images; authentication, verification and protection for copyright holdersxe2x80x9d (Digimarc Corp""s copyright protection technology), Aug. 14, 1995, Seybold Report on Desktop Publishing, v9, n. 12, p. 23(2).
xe2x80x9cNEC develops digital watermarking technique protecting copyrights of images and music on Internet,xe2x80x9d Feb. 12, 1996, xe2x80x94Business Wire, describes watermark information hidden in an image, using the spectral components of the data in a manner analogous to spread spectrum communications.
Accordingly, the inventor has identified a need in the hafltoning community for systems and methods for providing substantially invisible color watermarks in digitally reproduced color documents.
Various exemplary embodiments of the systems and methods according to this invention deal with a basic problem in providing substantially invisible watermarks in a digitally reproducible color document. In particular, in various exemplary embodiments of the systems and methods according to this invention the contrast of the colors of the watermark is enhanced to improve the watermark""s detectability while reducing the detectability of the watermark in the base image.
A stochastic screen can be used to produce an invisible watermark. A stochastic halftone screen is a large threshold array that produces a halftone image having an appearance with randomly distributed dots. To produce an invisible watermark for a stochastic screen, at least one additional stochastic halftone screen is produced and used to incorporate watermark information into the document. In the various exemplary embodiments described below, the number of stochastic halftone screens corresponds to the number of color separations forming the color document. However, the number of stochastic halftone screens that can be used is not limited to the number of color separations, since the extension to more, or even less, stochastic halftone screens than the number of color separations is straightforward for those skilled in the art.
This invention is directed to systems and methods that produce color watermarks in digitally reproducible color documents. In various exemplary embodiments of the systems and methods according to this invention, the color watermarks are generated by producing a halftone pattern, which appears as stochastically distributed dots, in at least one or more color separations of the color document. A second halftone pattern, which appears also as stochastically distributed dots, is generated in at least one or more color separations of the color document. Portions of the first and second halftone patterns are either auto-correlated or conjugately correlated. As a result, when the two halftone patterns are laid over each to become properly aligned, or otherwise appropriately aligned, the watermark patterns of highly contrasting colors become markedly visible.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.